<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>moon</title>
</head>
<body>
<div class="sky">
    <div class="moon"></div>
</div>
<style>
    .sky{
        width: 100vw;
        height: 100vh;
        background: #555555;
        margin: 0px;
        display: flex;
        align-items: center;
        justify-content: center;
    }
    .moon{
        width: 10em;
        height: 10em;
        border-radius: 50%;
        background: yellow;
        box-shadow: 0 0 10vmin 1vmin white;
    }
</style>
</body>
</html>